The most lethal of all the poisonous chemicals
used during the war, it was almost odourless and took twelve hours to take
effect. Yperite was so powerful that only small amounts had to be added to high
explosive shells to be effective. Once in the soil, mustard gas remained active
for several weeks.

The skin of victims of mustard gas blistered, the eyes became very sore and they
began to vomit. Mustard gas caused internal and external bleeding and attacked
the bronchial tubes, stripping off the mucous membrane. This was extremely
painful and most soldiers had to be strapped to their beds. It usually took a
person four or five weeks to die of mustard gas poisoning. One nurse, Vera
Brittain, wrote: "I wish those people who talk about going on with this
war whatever it costs could see the soldiers suffering from mustard gas
poisoning. Great mustard-coloured blisters, blind eyes, all sticky and stuck
together, always fighting for breath, with voices a mere whisper, saying that
their throats are closing and they know they will choke."